java核心技术卷的一个单线程小球移动

import javax.swing.;
import java.awt.;
import java.awt.event.ActionListener;

public class Bounce {

public static void main(String[] args) {

    EventQueue.invokeLater(() -> {
        JFrame frame = new BounceFrame();
        frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
        frame.setVisible(true);
    });
}

}

class BounceFrame extends JFrame {

private BallComponent comp;
public static final int STEPS = 1000;
public static final int DELAY = 3;

public BounceFrame() {

    setTitle("Bounce");
    comp = new BallComponent();
    add(comp, BorderLayout.CENTER);
    JPanel buttonPanel = new JPanel();
    addButton(buttonPanel, "Start", event -> addBall());
    addButton(buttonPanel, "Close", event -> System.exit(0));
    add(buttonPanel, BorderLayout.SOUTH);
    pack();
}

public void addButton(Container c, String title, ActionListener listener) {
    JButton button = new JButton(title);
    c.add(button);
    button.addActionListener(listener);
}

public void addBall() {

    try
    {
        Ball ball = new Ball();
        comp.add(ball);

        for (int i = 1; i <= STEPS; i++) {
            ball.move(comp.getBounds());
            comp.paint(comp.getGraphics());//重新绘制
            Thread.sleep(DELAY);//单线程延迟3毫秒
        }
    }
    catch (InterruptedException e)
    {

    }
}

}

import java.awt.geom.Ellipse2D;
import java.awt.geom.Rectangle2D;

public class Ball {

private static final int XSIZE = 15;
private static final int YSIZE = 15;
private double x = 0;
private double y = 0;
private double dx = 1;
private double dy = 1;

public void move(Rectangle2D bounds) {

    x += dx;
    y += dy;
    if (x < bounds.getMinX()) {
        x = bounds.getMinX();
        dx = -dx;
    }
    if (x + XSIZE >= bounds.getMaxX()) {
        x = bounds.getMaxX() - XSIZE;
        dx = -dx;
    }
    if (y < bounds.getMinY()) {
        y = bounds.getMinY();
        dy = -dy;
    }
    if (y + YSIZE >= bounds.getMaxY()) {
        y = bounds.getMaxY() - YSIZE;
        dy = -dy;
    }
}

public Ellipse2D getShape() {
    return new Ellipse2D.Double(x, y, XSIZE, YSIZE);
}

}

import javax.swing.;
import java.awt.;
import java.util.ArrayList;

public class BallComponent extends JPanel {

private static final int DEFAULT_WIDTH = 450;
private static final int DEFAULT_HEIGHT = 350;

private java.util.List<Ball> balls = new ArrayList<>();

public void add(Ball b) {
    balls.add(b);
}

public void paintComponent(Graphics g) {

    super.paintComponent(g);
    Graphics2D g2 = (Graphics2D)g;
    for (Ball b : balls) {
        g2.fill(b.getShape());
    }
}

public Dimension getPreferredSize() {
    return new Dimension(DEFAULT_WIDTH, DEFAULT_HEIGHT);
}

}